As energy diversification intensifies, alternative sources of energy such as biogas from landfills and digesters become more attractive. Landfill (LFG) and digester gas (DG) are both excellent sources of methane. Waste handlers and municipalities are increasingly using biogas generated at sewage treatment plants and solid waste landfills as a biofuel. Biogases can be used as a fuel to run internal combustion engines or turbines that power equipment or generate electrical power. However, these biogases can be contaminated by siloxanes, reduced sulfur compounds, and a variety of volatile organic compounds (VOC's).
Siloxanes are used in the manufacture of personal hygiene, health care and industrial products. As a consequence of their widespread use, siloxanes are found in wastewater and in solid residential and industrial waste deposited in landfills. At wastewater treatment plants and landfills, low molecular weight siloxanes can volatilize into digester gas and landfill gas. Although the amount of volatized siloxanes present in such gases may be small, the siloxanes are problematic. Siloxanes are nuisance contaminants that accumulate on energy generation equipment, resulting in increased maintenance and operating costs. When the biogas is combusted to generate power (such as in gas turbines, internal combustion engines or boilers), siloxanes are converted to silicon dioxide (SiO2), which can deposit in the combustion and/or exhaust stages of the equipment. The silica deposits can cause abrasive wear on engine parts and lead to reduced engine life. The deposits also can inhibit conduction and lubrication, and also can dislodge and clog lines, or contaminate engine oil. The siloxanes in biogases thus can significantly increase wear and other damage in equipment. Manufacturers of gas turbines and reciprocating engines are increasingly calling for the reduction of the presence of siloxanes in combustion gases in order to warranty their products. Waste digesters or landfills for which the biogas is used as fuel before making any emissions have a need to remove siloxanes and other materials that may deposit on the burners. Electrical power generators that use emission catalysts with combustion of biogases that contain siloxanes also are prone to incur silica film development with consequent reductions in catalytic activity, and also have siloxane removal needs.
Landfills that vent gases directly to the atmosphere can have emission control needs that go beyond siloxane removal. Landfill gases that will be vented to the atmosphere may need to be scrubbed before release to remove not only siloxanes, but also reduced sulfur compounds (e.g., H2S) and VOCs, and other contaminants that may be carried in the gas.
Sorbent media such as silica gel, layered beds, or bituminous coal and coconut activated carbon have been proposed for siloxane removal. Conventional siloxane removal strategies for biogas purification have tended to rely on multi-stage and/or multi-layer procedures including hydrogen sulfide (H2S) pre-treatment followed by treatment with adsorbent media for siloxane removal. In U.S. Pat. No. 7,264,648, filter beds formed of a plurality of different layers of different filter media are described therein as being useful for siloxane removal. Bituminous coal and coconut based activated carbon, for example, are widely used in the industry for removal of many types of contaminants. Silica gel behaves like a molecular sieve, and may only perform well when targeting a single contaminant under controlled conditions. Silica gel also can have low macro-porosity which can restrict diffusion, and silica gel may be vulnerable to contaminant spikes or composition variations. “Super Siloxane Adsorbent Silica gel” by ADCOA has been marketed for use in siloxane removal.
New strategies for siloxane removal from gas streams are needed, which can provide high adsorbent capacity for siloxanes without relying on designs that are complicated and/or costly. Also, new strategies are needed for single stage or single layer removal of siloxanes and different companion contaminants from the same gas streams.